The present invention relates to a brake system including a brake booster which boosts a braking liquid pressure, and more particularly, to a brake system which permits a boosting operation after the brake booster has reached a full load point so as to maintain the master cylinder pressure as before the full load point is reached.
A brake system is known in the art which permits a braking liquid pressure to be boosted even after a brake booster has reached a full load point (see Japanese Laid-Open Patent Application No. 152,041/1998). This cited Application discloses an embodiment shown in FIGS. 2 and 3 of the cited application in which a solenoid operated pressure control valve 22 and a pump 16 are provided in a braking liquid passage extending between a master cylinder and a wheel cylinder so that a liquid pressure from the pump 16 is acting in a portion of the braking liquid passage located nearer the wheel cylinder than the pressure control valve. This allows the braking liquid pressure to be increased relative to a brake operating force, enabling the braking force to be boosted after the full load point of, or the limit of assistance by the brake booster. This also assures that a required braking force can be obtained in the event the brake booster fails to operate as a result of a drop in the pressure of a source of pressure for the brake booster.
The above cited Application also discloses an embodiment in FIGS. 13 and 14 of the cited application in which the solenoid operated pressure control valve 22 shown in FIGS. 2 and 3 of the cited application is replaced by a mechanical pressure control valve 150. Again, a boosting of a braking force is enabled subsequent to the full load point of the brake booster.
A brake system which permits the braking liquid pressure to be boosted subsequent to the braking booster reaching the full load point is also known from Japanese Laid-Open Patent Application No. 35,477/1998, which discloses a system including, in addition to a master cylinder and a brake booster, an intermediate liquid pressure controller 154 for use when the limit of assistance is reached, the controller being disposed rearward of the master cylinder for enabling a boosting of a braking force subsequent to the full load point of the brake booster.
The system disclosed in FIGS. 2 and 3 of the first cited Application requires a variety of sensors and a solenoid operated pressure control valve, and disadvantageously results in a complicated and expensive arrangement. The system also disclosed in FIGS. 13 and 14 of the same Application suffers from a drawback that the assistance is prohibited if the brake booster fails to operate due to a drop in the pressure of a source of pressure for the brake booster.
The brake system disclosed in the second cited Application also suffers from the same drawback that the assistance is prohibited if the brake booster fails to boost due to a drop in the pressure of a source of pressure for the brake booster.
It is an object of the present invention to provide a brake system which uses a relatively simple arrangement to permit a braking force to be boosted subsequent to the full load point of or the limit of assistance by the brake booster and to enable the assistance if the brake booster fails to boost due to a depression or drop in the pressure of a source of pressure for the brake booster.
Specifically, the present invention provides a brake system comprising a master cylinder defined in a housing for developing a master cylinder pressure as an associated master cylinder piston disposed therein advances, a negative pressure booster actuated by a brake operating member to develop an output which in turn causes the master cylinder piston to advance, and intensifying means for intensifying the master cylinder pressure by causing the master cylinder piston to advance, the arrangement being such that a sum of a force with which the intensifying means urges the master cylinder piston to advance and a force with which the output from the negative pressure booster urges the master cylinder piston to advance is in proportion to an input to the brake operating member.
With this arrangement, it is possible to develop a master cylinder pressure after the negative pressure booster has reached the full load point or the limit of assistance at the same ratio as before the full load point is reached. If a source of pressure for the brake booster fails and the brake booster fails to boost, a master cylinder pressure can be developed in the similar manner as when the source of pressure is normal.
In this manner, a brake system can be provided which uses a relatively simple arrangement to allow a braking force to be boosted subsequent to the full load point or the limit of assistance of the brake booster while also enabling the assistance if the brake booster fails to boost due to a depression in the pressure of a source of pressure for the brake booster.
Above and other objects, features and advantages of the invention will become apparent from the following description of several embodiments thereof with reference to the attached drawings.